The invention relates to investment casting. More particularly, it relates to the investment casting of superalloy turbine engine components.
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The invention is described in respect to the production of particular superalloy castings, however it is understood that the invention is not so limited.
Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In gas turbine engine applications, efficiency is a prime objective. Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures in the turbine section exceed the melting points of the superalloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections.
The cooling passageway sections may be cast over casting cores. Ceramic casting cores may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened steel dies. After removal from the dies, the green cores are thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile. Commonly-assigned U.S. Pat. No. 6,637,500 of Shah et al. and U.S. Pat. No. 6,929,054 of Beals et al (the disclosures of which are incorporated by reference herein as if set forth at length) disclose use of ceramic and refractory metal core combinations.
FIG. 1 shows a trailing edge portion of a turbine airfoil 20 as cast within a shell 22. For casting the internal passageways, the shell contains a core assembly. The exemplary core assembly includes a ceramic feed core having spanwise legs 30, 32, and 34 for casting associated passageway legs. The leg 34 casts a trailing spanwise passageway 36. The core assembly also includes metallic cores, of which cores 40, 42, and 44 are shown. The exemplary metallic cores are formed of refractory metal sheet stock. The core 40 forms a pressure side outlet circuit, the core 42 forms a suction side outlet circuit, and the core 44 forms a trailing edge outlet slot 50. The outlet slot 50 is fed from the passageway 36. During core assembly, a leading portion of the core 44 is secured within a mating slot of the trailing leg 34 of the ceramic core. With such a configuration, the transition between the passageway 36 and the outlet slot 50 may be relatively abrupt and may create relatively thick areas 52 and 54 of the pressure and suction side walls.